The following U.S. Patent publications are believed to represent the current state of the art: U.S. Pat. Nos. 7,099,553; 6,972,401; 6,783,269; 5,257,340; 5,905,583; 7,163,326 and 5,295,047, and U.S. Published Patent Applications 2005/0248540 and 2005/0128190.